Relativistic stars in beyond Horndeski theories

Eugeny Babichev, Kazuya Koyama, David Langlois, Ryo Saito, Jeremy Sakstein

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Abstract

This work studies relativistic stars in beyond Horndeski scalar-tensor theories that exhibit a breaking of the Vainshtein mechanism inside matter, focusing on a model based on the quartic beyond Horndeski Lagrangian. We self-consistently derive the scalar field profile for static spherically symmetric objects in asymptotically de Sitter space-time and show that the Vainshtein breaking branch of the solutions is the physical branch thereby resolving several ambiguities with non-relativistic frameworks. The geometry outside the star is shown to be exactly Schwarzschild-de Sitter and therefore the PPN parameter βPPN =1, confirming that the external screening works at the post-Newtonian level. The Tolman-Oppenheimer-Volkoff (TOV) equations are derived and a new lower bound on the Vainshtein breaking parameter ϒ> -4/9 is found by requiring the existence of static spherically symmetric stars. Focusing on the unconstrained case where ϒ1 < 0 we numerically solve the TOV equations for polytropic and realistic equations of state and find stars with larger radii at fixed mass. Furthermore, the maximum mass can increase dramatically and stars withmasses in excess of 3M Θ can be found for relatively small values of the Vainshtein breaking parameter. We re-examine white dwarf stars and show that post-Newtonian corrections are important in beyond Horndeski theories and therefore the bounds coming from previous analyses should be revisited.
Original languageEnglish
JournalClassical and Quantum Gravity
Volume33
Issue number23
DOIs
Publication statusPublished - 8 Nov 2016

Keywords

  • gr-qc
  • astro-ph.CO
  • hep-th

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